Section 4: Mercury Cell Chlor-Alkali Industry

• 4.1 Canadian-Oxy Ltd., Squamish, British Columbia
• 4.2 ICI Ltd., Cornwall, Ontario
• 4.3 ICI Ltd., Dalhousie, New Brunswick
• 4.4 Canso Chemicals Ltd., Point Abercrombie, Nova Scotia
• 4.5 PPG Canada Inc., Beauharnois, Quebec
• 4.6 Mercury Loss to Effluents and Emissions
• 4.7 Mercury Loss to Products and Solid Wastes
• 4.8 Mercury Accountability

During the 1986-1989 period covered by this report, there were five operational mercury cell chlor-alkali plants in Canada. Table 9 summarizes relevant operational information for each of these plants as of 1989. This section summarizes each mercury cell plant's adherence to the requirements of the federal liquid effluent and emissions regulations.

The plant is located on tidewater at the head of Howe Sound on the western bank of Maquam Blind Channel. Final effluent from the plant is discharged into Howe Sound.

Canadian-Oxy produces NaOH, Cl₂, and HCl, which are used primarily by the pulp and paper industry.

Deposits of mercury in effluent from the plant for 1986-1989 are presented in Table 10. During this 4 year period, there were no violations of the federal chlor-alkali effluent regulations.

A distinguishing feature of this plant is that the mercury cells are not contained within a building but are open to the atmosphere. Consequently, mercury emissions from the cells are released directly to the atmosphere and are difficult to measure using the methodology specified in the standard reference for mercury emissions. Cell emissions front this plant, therefore, are calculated from quarterly monitoring of ambient mercury concentration (μg/m³) and air flow rate (m³/min) at six locations in the cell area to give a mass/unit time of mercury released. Mercury emissions from Canadian-Oxy for 1986-1989 are presented in Table 11.

For 1987, total mercury emissions from the cell area were 466.3 kg, above the allowable limit of 335.8 kg/year based on the rated capacity. This excess, emitted during the third quarter of 1987, was caused by a combination of several factors:

• the plant was operating at its highest recorded production levels due to increased electrical capacity and good market conditions
  
• high overall cell temperatures (caused by high production levels)
  
• high ambient temperatures (caused by hot summer weather)
  
• seawater cooling deficiencies
  
• equipment malfunctions

Following discussions with federal and provincial regulatory authorities, the company immediately improved its housekeeping practices and modified process equipment, pumps, and tanks to improve sealing and prevent mercury emissions from the cell area.

Modifications included the following:

• new mercury pump tank covers were made of fibre-reinforced plastic (FRP)
  
• mercury pump tanks were upgraded to improve the seal between the cover and the tank
  
• a new head end-box wash-water system was installed (to improve cooking and reduce emissions)
  
• high-performance shaft seals were installed on the mercury pumps and solid mountings were used to prevent mercury entrapment
  
• mercury level indicators were installed on the mercury pump tanks to minimize mercury leaks
  
• end-box venting was improved by resloping the vent lines
  
• the liquid drain seal of the heat exchanger was deepened to allow operating at a higher vacuum without re-entrainment of condensed liquids (re-entrainment caused pressure fluctuations that produced puffing of mercury from the mercury pump tanks)
  
• decomposer lids were replaced with an improved design that reduced the number of leak points

These modifications were started during the fourth quarter of 1987 and were completed in 1988. As a result, mercury emissions during the fourth quarter of 1987 were lower, reflecting the process modifications and equipment improvements. For 1988 and 1989, mercury emissions were well below allowable levels, again reflecting the process modifications and equipment improvements.

With a cell area that is not enclosed, it is difficult to state exactly how many days in 1987 the cell area at Canadian-Oxy was not in compliance. Theoretically, the plant was not in compliance for the full year, except during the period of annual maintenance.

Mercury emissions from the end box and hydrogen stream were below allowable limits and, therefore, in compliance over the entire 1986-1989 period. The retorts have not been in service since 1983.

Losses of mercury to products and solid wastes for 1986-1989 are presented in Table 12. A summary of all operating mercury losses (effluent, emissions, products, and solid wastes), total chlorine production, and loss of mercury per tonne of chlorine produced is presented in Table 13.

The mercury cell chlor-alkali plant at Cornwall is the oldest plant of its kind in Canada, beginning operations in 1935. Subsequent expansions in 1954/1955 resulted in a plant size of 60 cells, producing (in addition to chlorine, caustic soda, and hydrogen) potassium hydroxide, HCI, and NaOCI. Products from the chlor-alkali operation are used at an on-site facility to produce carbon disulphide and carbon tetrachloride.

Deposits of mercury in effluent from the plant for 1986-1989 are presented in Table 14. During this 4 year period, there were no violations of the federal chlor-alkali effluent regulations. On one day in April 1988, however, the plant registered mercury in the effluent at a level of 98 g above the permitted daily discharge of 375 g. Upon investigation by Environment Canada personnel, it was found that the plant had accumulated this effluent for a 2-3 d period. If this is done, regulations allow averaging of the mercury in the effluent over the number of days during which it accumulated (Section 10(2)). When averaged in this manner, the final mercury level was in compliance. Although prosecuted, the company was found not guilty of violating effluent regulations.

Mercury emissions from ICI, Cornwall, for 1986-1989 are presented in Table 15. As indicated, the company was in compliance over the entire period.

Losses of mercury to products and solid wastes for 1986-1989 are presented in Table 16. A summary of all operating mercury losses, total chlorine production, and loss of mercury per tonne of chlorine produced is presented in Table 17.

Production of chlorine and caustic soda using the mercury cell process began at ICI, Dalhousie, in 1963. Plant expansion in 1965 resulted in its present size of 38 cells and rated capacity of 94 tonnes of chlorine per day. Treated process effluent from the plant is discharged into Chaleur Bay.

Deposits of mercury in effluent from the plant for 1986-1989 are presented in Table 18. During this 4 year period, there were no violations of the federal chlor-alkali effluent regulations.

Mercury emissions from ICI, Dalhousie, for 1986-1989 are presented in Table 19.

In 1986, during the third quarter compliance tests, there were high mercury emissions from the cell room. These emissions resulted from a lack of plant maintenance during the summer and high plant output. Cell room emissions were reduced by taking sortie cells off-load, overhauling cells, repairing leaks, and general cleaning. Consequently, when the quarterly compliance tests for 1986 were averaged, the plant was found to be in compliance. Cell room emissions were in compliance for 1987-1989 inclusive.

In addition to the high mercury emissions from the cell room, mercury emissions from the hydrogen stream were not in compliance during the following periods:

1. January and February 1986 due to problems with the turbulent contact absorption scrubber,
   
2. 1986 third quarter compliance tests due to high plant output and a lack of plant maintenance (emissions were reduced by reducing output, overhauling cells, and repairing leaks), and
   
3. 1987 third quarter compliance tests due to problems with the scrubber.

Following discussions with headquarter and regional personnel, ICI, Dalhousie, improved its housekeeping practices by increasing the maintenance cycle on the cell, using the mercury analyzer on a daily basis to detect problem areas in the cell room, and encouraging plant operators to maintain low emissions through an incentive plan.

Losses of mercury to products and solid wastes for 1986-1989 are presented in Table 20.

A summary of all operating mercury losses, total chlorine production, and loss of mercury per tonne of chlorine produced is presented in Table 21.

Canso Chemicals commenced operations in 1970 and in 1971 underwent a 50% production expansion, a level maintained to the present. Canso Chemicals, jointly owned by ICI Canada Inc., Scott Paper Ltd., and Stora Forest Products Ltd., produces chlorine, caustic soda, and hydrogen. Chlorine, caustic soda, and hydrogen are piped directly to Scott Paper, on whose property Canso Chemicals is situated.

Solid waste produced at Canso Chemicals is separated into recoverable mercury and nonrecoverable mercury fractions. The nonrecoverable fraction, e.g., brine sludge, contains less than 10 ppm mercury and is buried in an on-site landfill. The landfill site is lined with clay, bas monitoring wells, and is a licensed site. The recoverable fraction, e.g., caustic sludge and cell room muds, contains mercury ranging in concentration from several thousand parts per million to percent levels and is stared on-site in drums lined with plastic for recovery of the mercury at a later date.

Deposits of mercury in effluent from the plant for 1986-1989 are presented in Table 22.

Mercury emissions from Canso Chemicals for 1986-1989 are presented in Table 23.

Losses of mercury to products and solid wastes for 1986-1989 are presented in Table 24. A summary of all operating mercury losses, total chlorine production, and loss of mercury per tonne of chlorine produced is presented in Table 25.

The first mercury cell plant at Beauharnois was built in 1948, commenced operations in 1949 as Standard Chemicals Ltd., and operated until 1959. In 1955, the plant was purchased by Pittsburgh Plate and Glass Inc. (PPG), but retained its original name until 1977 when it was changed to Stanchem. In 1984, the plant's name was changed again to the present name of PPG Canada Inc.

In 1959, the first mercury cell plant was closed and a second mercury cell plant started operating. This plant continued operating until November 1990, at which time it was permanently closed. At the same time, a new membrane cell plant started operations.

An effluent treatment plant was built in 1973 to treat mercury-bearing plant process water. A new wastewater treatment plant began operating in 1986.

Currently, a site clean-up project is under way to restore the Beauharnois site to as near a mercury-free condition as possible. The clean-up involves plant restoration (removal of mercury-contaminated soils for treatment and burial), removal of all former cell house structures (by demolition and landfilling), and landfill restoration (excavating the three oldest on-site landfills and placing this material in a new, monitored, state-of-the-art landfill). The total cost of the mercury removal and site-restoration project is estimated to be about $25 million.

Deposits of mercury in effluent from the plant for 1986-1989 are presented in Table 26. During this 4 year period, there were several violations of the federal chlor-alkali effluent regulations in 1986, none in 1987, three in 1988, and none in 1989. These violations, their causes, and remedial action taken are presented in Table 27.

Mercury emissions from PPG Canada for 1986-1989 are presented in Table 28. These values were compiled from company data submitted to Environment Quebec.

Only during 1988 were all sources in compliance with emission regulations. For the remaining years, compliance and noncompliance for emission sources are presented in Table 29.

Losses of mercury to products and solid wastes for 1986-1989 are presented in Table 30. A summary of all operating mercury losses, total chlorine production, and loss of mercury per tonne of chlorine produced is presented in Table 31.

Mercury loss to effluents and emissions for 1986-1989 is presented in Tables 32 and 33 respectively.

From 1986 to 1989, mercury loss to effluents consistently decreased from 88.07 kg to 46.29 kg, representing an overall decrease of 47%.

From 1986 to 1989, mercury loss to emissions decreased from 680.56 kg to 547.18 kg, representing an overall decrease of 20%. In 1987, although an emission level of 831.45 kg was recorded, this was due to a high mercury release from one plant (Canadian-Oxy Ltd.) during that year.

Apart from the loss of mercury to effluent streams and as plant emissions, the two remaining sources of mercury loss from chlor-alkali plants are to products (produced from chlorine, sodium hydroxide, and hydrogen) and as solid wastes generated during plant operations.

Because of the presence of small amounts of mercury in hydrogen and caustic soda (traces of which remain after product purification), when these chemicals are used to manufacture other products, such as sodium hypochlorite and hydrochloric acid, the new products contain traces of mercury. Although this may be regarded as product contamination, mercury concentrations remain within product specifications. Table 34 presents mercury losses to products for the years 1986-1989 for the five mercury cell plants.

The main sources of solid wastes produced at mercury cell plants are

• brine saturator sludges
  
• recycled brine sludges
  
• caustic soda product
  
• wastewater treatment plant sludge
  
• waste solids produced as a result of cell maintenance and retort use

For those plants employing some form of solid waste treatment for mercury recovery, the final product is an inert mercury-free solid (residue) that is landfilled. Monitoring wells are installed and sampled to determine if there is any migration of mercury to the groundwater.

Table 35 presents mercury losses to solid wastes for the years 1986-1989 for the five mercury cell plants.

One requirement of the chlor-alkali effluent regulations is that plants must report a yearly mercury inventory. This inventory consists of

1. Annual net input of mercury to the plant: This includes mercury purchases, mercury sales, and mercury transfers into and out of the plant.
   
2. Initial mercury inventory: This includes mercury in cell loops (i.e., cell, denuder, and end boxes), mercury in process equipment (i.e., traps, lines, tanks, sales, process stock tanks, and trench traps), recoverable mercury in solids that are scheduled for recovery (e.g., caustic soda filter media, denuder packing, carbon from effluent treatment, and mercury sulphides from effluent treatment), cell washing, cell floor pickup and washing, and mercury butter.
   
3. Final mercury inventory: This includes mercury from the same sources as for the initial mercury inventory. Note that initial and final mercury inventories are determined, respectively, at the beginning and end of an operating year.
   
4. Mercury disposition: This includes mercury loss in products (which leave the plant), liquid effluent, air emissions, and impounded solids (i.e., solids not scheduled for mercury recovery).

From this mass balance approach, yearly mercury consumption (or accountability) and variance at a plant may be determined from the following formulae:

Mercury consumption = (net input of mercury to the plant) + (any decrease in inventory)

Variance = (mercury consumption) - (total mercury disposition)

Variance in the mercury balance may result from: inherent difficulties and inaccuracies in measurements or estimates of inventories, nonrepresentative sampling, accumulation of nonrecoverable mercury within the plant (i.e., amalgamation to steel, penetration of concrete), and other unidentified or unidentifiable losses (i.e., theft, fugitive trackout from the cell room, etc.). Ideally, the variance should be zero.

Mercury accountability for the five chlor-alkali plants for 1986-1989 is presented in Tables 36-40.